With regards to his 3rd Meditation and 5th Meditation proofs of God, I can't seem to find any reason to see one as being more successful than the other.
Mainly because while his ontological proof rests on his assumption of our having clear and distinct perceptions, this in turn depends on his first argument of God’s existence, which depends on us sharing an innate idea of a perfect infinite God.
The two arguments seem to interact such that clear and distinct perceptions are made certain because God exists, and God's existence, as an essential property, is clearly and distinctly perceived. However, this only reinforces the Cartesian circle.
So the premises of his fifth meditation argument depend upon assumptions he previously makes in his trademark argument (that innate knowledge of God implies His existence) so both arguments seem to be equally refutable when we bring doubt to the premises used to justify the doctrine of innate ideas and of clear and distinct perceptions.
I would be grateful if anyone could point out to me any strengths/weaknesses of either argument that would help me in some way to decide which proof is more compelling (even if they are both poor!).